Ionizing radiation (IR) induces a variety of DNA damage of which double strand breaks (DSBs) are the most cytotoxic. These DSBs can lead to cell death via apoptosis and/or mitotic catastrophe if not rapidly and completely repaired. In addition to IR, certain chemotherapeutic agents including topoisomerase II inhibitors, bleomycin, and doxorubicin also cause DSBs. These DNA lesions trigger a complex set of signals through the DNA damage response network that function to repair the damaged DNA and maintain cell viability and genomic stability. In mammalian cells, the predominant repair pathway for DSBs is the Non-Homologous End Joining Pathway (NHEJ). This pathway functions regardless of the phase of the cell cycle and does not require a template to re-ligate the broken DNA ends. NHEJ requires coordination of many proteins and signaling pathways. The core NHEJ machinery consists of the Ku70/80 heterodimer and the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs), which together comprise the active DNA-PK enzyme complex. DNA-PKcs is a member of the phosphatidylinositol 3-kinase-related kinase (PIKK) family of serine/threonine protein kinases that also includes ataxia telangiectasia mutated (ATM), ataxia telangiectasia and Rad3-related (ATR), mTOR, and four PI3K isoforms. However, while DNA-PKcs is in the same protein kinase family as ATM and ATR, these latter kinases function to repair DNA damage through the Homologous Recombination (HR) pathway and are restricted to the S and G2 phases of the cell cycle. While ATM is also recruited to sites of DSBs, ATR is recruited to sites of single stranded DNA breaks.
NHEJ is thought to proceed through three key steps: recognition of the DSBs, DNA processing to remove non-ligatable ends or other forms of damage at the termini, and finally ligation of the DNA ends. Recognition of the DSB is carried out by binding of the Ku heterodimer to the ragged DNA ends followed by recruitment of two molecules of DNA-PKcs to adjacent sides of the DSB; this serves to protect the broken termini until additional processing enzymes are recruited. Recent data supports the hypothesis that DNA-PKcs phosphorylates the processing enzyme, Artemis, as well as itself to prepare the DNA ends for additional processing. In some cases DNA polymerase may be required to synthesize new ends prior to the ligation step. The auto-phosphorylation of DNA-PKcs is believed to induce a conformational change that opens the central DNA binding cavity, releases DNA-PKcs from DNA, and facilitates the ultimate religation of the DNA ends.
It has been known for some time that DNA-PK−/− mice are hypersensitive to the effects of IR and that some non-selective small molecule inhibitors of DNA-PKcs can radiosensitize a variety of tumor cell types across a broad set of genetic backgrounds. While it is expected that inhibition of DNA-PK will radiosensitize normal cells to some extent, this has been observed to a lesser degree than with tumor cells likely due to the fact that tumor cells possess higher basal levels of endogenous replication stress and DNA damage (oncogene-induced replication stress) and DNA repair mechanisms are less efficient in tumor cells. Most importantly, an improved therapeutic window with greater sparing of normal tissue will be imparted from the combination of a DNA-PK inhibitor with recent advances in precision delivery of focused IR, including image-guide RT (IGRT) and intensity-modulated RT (IMRT).
Inhibition of DNA-PK activity induces effects in both cycling and non-cycling cells. This is highly significant since the majority of cells in a solid tumor are not actively replicating at any given moment, which limits the efficacy of many agents targeting the cell cycle. Equally intriguing are recent reports that suggest a strong connection between inhibition of the NHEJ pathway and the ability to kill traditionally radioresistant cancer stem cells (CSCs). It has been shown in some tumor cells that DSBs in dormant CSCs predominantly activate DNA repair through the NHEJ pathway; it is believed that CSCs are usually in the quiescent phase of the cell cycle. This may explain why half of cancer patients may experience local or distant tumor relapse despite treatment as current strategies are not able to effectively target CSCs. A DNA-PK inhibitor may have the ability to sensitize these potential metastatic progenitor cells to the effects of IR and select DSB-inducing chemotherapeutic agents.
Given the involvement of DNA-PK in DNA repair processes, an application of specific DNA-PK inhibitory drugs would be to act as agents that will enhance the efficacy of both cancer chemotherapy and radiotherapy. Accordingly, it would be desirable to develop compounds useful as inhibitors of DNA-PK.